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I'd start by doing a search for
performance tuning, as this question is asked many times with generally very similar answers. In a nutshell you need look at how your webserver is configured to run PHP (CGI sux for speed, but is the way most default installs and shared hosting providers run it), install a PHP opcode cache (such as eAccelerator/APC), look at optimising your MySQL caching settings and finally configure you XOOPS caching and ensure that your theme is not bloated with images or bad HTML.
If you are using a shared host, then unfortnately it is quite likely that you will not have access to configure any of these apart from your XOOPS caching and theme. You can try asking your host about this, but if they are not prepared to come to the party then I would recommend changing hosts.